Precoding a Transmission from a Multi-Panel Antenna Array

ABSTRACT

The invention relates to a wireless communication device configured for use in a wireless communication system, wherein, based on one or more structural properties of a multi-panel antenna array describing how the antenna array is structured into multiple panels, a precoder is selected to be applied for a transmission from the multi-panel antenna array; and wherein an information indicative of the determined precoder is signaled to a transmit radio node; the invention further refers to a transmit radio node configured for transmitting via a multi-panel antenna array in a wireless communication system, wherein signaling indicating one or more structural properties of a multi-panel antenna array describing how the antenna array is structured into multiple panels is transmitted to the wireless communication device.

RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 16/475,401, which was filed on Jul. 2, 2019, which is anational stage application of PCT/EP2017/083992, which was filed Dec.21, 2017, and claims benefit of U.S. Provisional Application 62/443,453,which was filed Jan. 6, 2017, the disclosures of each of which areincorporated herein by reference in their entirety.

BACKGROUND

Precoding a transmission from a transmit antenna array involves applyinga set of complex weights to the signals that are to be transmitted fromthe array's antenna elements, so as to independently control thesignals' phase and/or amplitude. This set of complex weights is referredto as a “precoder” or “precoding matrix”. The transmitting radio nodeconventionally chooses the precoder to match the current channelconditions on the link to the receiving radio node, with the aim ofmaximizing the link capacity or quality. If multiple data streams aresimultaneously transmitted from the array's antenna elements usingspatial multiplexing, the transmitting radio node also typically choosesthe precoder with the aim of orthogonalizing the channel and reducinginter-stream interference at the receiving radio node.

In closed-loop operation, the transmitting radio node selects theprecoder based on channel state information (CSI) fed back from thereceiving radio node that characterizes the current channel conditions.The transmitting radio node in this regard transmits a reference signalfrom each antenna element to the receiving radio node, and the receivingradio node sends back CSI based on measurement of those referencesignals.

Transmission of the CSI feedback threatens to contribute significantoverhead to precoding schemes. Known approaches address overheadattributable to CSI feedback by limiting the usable precoders to a fixedset of precoders, i.e., a codebook. Each precoder in the codebook isassigned a unique index that is known to both the transmitting node andthe receiving node. The receiving node determines the “best” precoderfrom the codebook, and feeds back the index of that precoder (oftenreferred to as a “precoding matrix indicator”, PMI) to the transmittingnode as a recommendation (which the transmitting node may or may notfollow). Feeding back only an index, in conjunction with other CSI suchas the recommended number of data streams (i.e., transmission rank) forspatial multiplexing, reduces the number of transmission resourcesrequired for transporting that CSI. This approach therefore reduces CSIfeedback overhead considerably.

Known approaches design precoder codebooks for CSI feedback withsingle-panel antenna arrays in mind that fit all hardware components ofthe array into a single panel.

SUMMARY

One or more embodiments herein provide precoding tailored formulti-panel antenna arrays. Precoding may for instance account for thedifferent structural properties that different types of multi-panelantenna arrays may have in terms of how the antenna arrays arestructured into multiple panels (e.g., in terms of the number and/orspatial arrangement of the panels). Precoding recommendations from awireless communication device may therefore likewise be based, at leastimplicitly, on one or more structural properties of the multi-panelantenna array from which the device is to receive a precodedtransmission. In some embodiments, for instance, a codebook from whichprecoder recommendations (or actual precoders) are chosen accounts for,is tailored for, or is otherwise chosen based on the multi-panel antennaarray's structural properties. Indeed, in one such embodiment, thedevice receives signaling from a transmit radio node indicating one ormore of these structural properties, determines (e.g., calculates orselects) a precoding codebook tailored for those structural properties,and chooses a precoder to recommend from that codebook. With a precodercodebook structure tailored towards multi-panel antenna arrays, coherenttransmission from the multi-panel antenna array may be realized, even inthe face of non-uniform and/or uncalibrated panels.

In some embodiments, tailoring precoding (e.g., in terms of codebookdesign) for the multi-panel nature of multi-panel antenna arrays provesadvantageous in that it improves the quality of precoding as well asprecoder recommendations (and CSI reports in general), e.g., as comparedto precoding schemes designed implicitly assuming a single-panel array.This is especially true for Discrete Fourier Transform (DFT) precoding,as applying a DFT precoder across multiple antenna panels may lead topoor precoding performance, especially if the antenna panels areuncalibrated with respect to one another and/or mismatched in terms oftheir antenna element spacing.

More particularly, embodiments herein include a method performed by awireless communication device configured for use in a wirelesscommunication system. The method comprises determining, based on one ormore structural properties of a multi-panel antenna array describing howthe antenna array is structured into multiple panels, a precoder torecommend to a transmit radio node for applying to a transmission fromthe multi-panel antenna array. In some embodiments, the method alsocomprises receiving signaling indicating the one or more structuralproperties. Regardless, the method may also comprise signaling thedetermined precoder to the transmit radio node.

Embodiments herein also include a method implemented by a transmit radionode configured for transmitting via a multi-panel antenna array in awireless communication system. The method comprises transmitting to awireless communication device signaling indicating one or morestructural properties of a multi-panel antenna array describing how theantenna array is structured into multiple panels. In some embodiments,the method also comprises, responsive to transmitting that signaling,receiving from the wireless communication device signaling indicating aprecoder that the wireless communication device recommends to thetransmit radio node for applying to a transmission from the multi-panelantenna array.

Embodiments moreover include corresponding apparatus, computer programs,and computer readable storage mediums.

FIG. 1 illustrates a wireless communication system according to one ormore embodiments,

FIG. 2 illustrates a processing performed by a device of the wirelesscommunication system according to some embodiments,

FIG. 3 illustrates processing performed by the transmit radio node ofthe wireless communication system according to some embodiments,

FIG. 4 shows an exemplary structural block diagram of the wirelesscommunication device according to some embodiments,

FIG. 5 shows an exemplary functional block diagram of the wirelesscommunication device according to some embodiments,

FIG. 6 shows an exemplary structural block diagram of the transmit radionode according to some embodiments,

FIG. 7 shows an exemplary functional block diagram of the transmit radionode according to some embodiments,

FIG. 8 illustrates a spatial multiplexing operation,

FIG. 9 shows an example of an antenna array with 4×4 cross-polarizedantenna elements,

FIG. 10 shows an example of a 2×2 multi-panel antenna array with 4cross-polarized panels, and

FIG. 11 shows exemplary precoders forming a grid of DFT beams.

DETAILED DESCRIPTION

FIG. 1 illustrates a wireless communication system 10 according to oneor more embodiments. The system 10 includes a transmit radio node 12,shown in the form of a base station, that performs transmissions via amulti-panel antenna array 14. The array 14 is structured into multiplepanels 14-1, 14-2, . . . 14-N. The panels may be spatially arranged in asingle spatial dimension (e.g., stacked vertically or alignedhorizontally) or multiple spatial dimensions. Each panel 14-n in turnhas one or more antenna elements arranged in one or more spatialdimensions.

The transmit radio node 12 may perform a transmission 18 via the array14 by feeding one or more signals of the transmission 18 to one or moreantenna elements of the array 14, respectively. The radio node 12 insome embodiments independently controls the amplitude and/or phase ofthe signal(s) fed to the array's antenna element(s), as part ofprecoding the transmission 18 from the array 14. The radio node 12 inthis regard applies a precoder (e.g., a precoding matrix) to thetransmission 18. The radio node 12 may select the precoder to which toapply to the transmission 18 based on a recommendation that the wirelessdevice 16 feeds back to the radio node 12 via signaling 20. Notably,some embodiments herein tailor the radio node's precoding and/or thedevice's precoder recommendation to the structural properties of theparticular multi-panel antenna array 14 via which the precodedtransmission is transmitted, e.g., in terms of how the array 14 isstructured into multiple panels. FIG. 2 illustrates processing performedby the device 16 according to some embodiments in this regard.

As shown in FIG. 2, processing at the device 16 includes determining,based on one or more structural properties of the multi-panel antennaarray 14 describing how the antenna array 14 is structured into multiplepanels 14-1, 14-2, . . . 14-N, a precoder to recommend to the transmitradio node 12 for applying to a transmission 18 from the multi-panelantenna array 14 (Block 100). The one or more structural properties mayinclude, for example, the total number of array's panels, the number ofpanels in each of one or more spatial dimensions in which the panels arearranged, and/or a spatial arrangement of the panels. Regardless,processing also includes signaling the determined precoder to thetransmit radio node 12 (Block 110).

In some embodiments, the device 16 is able to base the recommendedprecoder on these one or more structural properties because the device16 receives signaling 22 (e.g., from the transmit radio node 12 or someother node) indicating those one or more structural properties. Indeed,in at least some embodiments, the device 16's mobile nature means thatthe device 16 may (simultaneously or at different times) receivetransmissions from different types of antenna arrays, including evendifferent types of multi-panel antenna arrays that may be structuredinto multiple panels in different ways (e.g., with a different number ofpanels and/or spatial arrangement of the panels). This signaling 22 maytherefore inform the device about the structural properties of theparticular multi-panel antenna array from which the device 16 is toreceive a transmission 18. FIG. 3 illustrates processing performed bythe transmit radio node 12 in this regard.

As shown in FIG. 3, processing by the transmit radio node 12 may includetransmitting to the wireless communication device 16 signaling 22indicating one or more structural properties of the multi-panel antennaarray 14 describing how the antenna array 14 is structured into multiplepanels (Block 200). Again, the one or more structural properties mayinclude, for example, the total number of array's panels, the number ofpanels in each of one or more spatial dimensions in which the panels arearranged, and/or a spatial arrangement of the panels.

This signaling 22 may be radio resource control (RRC) signaling, mediumaccess control (MAC) signaling, or physical layer signaling. In oneembodiment, for instance, the signaling 22 is transmitted or receivedduring a procedure for configuring a channel state information (CSI)process or a CSI reference signal (CSI-RS) resource. In otherembodiments, though, the signaling may be included in a downlink controlinformation (DCI) message on a physical downlink control channel. Forinstance, the DCI message may convey scheduling information, e.g., inthe form of an uplink grant for the device 16 to transmit a CSI feedbackreport to the transmit radio node 12.

In any event, processing at the transmit radio node 12 in someembodiments may further include, responsive to transmitting thesignaling 22, receiving from the wireless communication device thesignaling 20 indicating a precoder that the wireless communicationdevice 16 recommends to the transmit radio node 12 for applying to thetransmission 18 from the multi-panel antenna array 14 (Block 210). Asnoted above, because the device 16 bases its determination of therecommended precoder on the structural properties of the array 14, therecommended precoder reflects the one or more structural properties thatthe radio node 12 indicated to the device 16 via signaling 22.

Processing may then entail determining the actual precoder to apply tothe transmission 18, based on the precoder recommended by the device 16(e.g., the recommended precoder may or may not be actually used, at thetransmit radio node's discretion). Processing may next entail precodingthe transmission 18 from the array 14 using the determined precoder, andtransmitting the transmission 18.

Where codebook-based precoding is used, tailoring precoding for thearray's panel-related structural properties may involve using a codebookthat accounts for, is designed or tailored for, or is otherwise chosenbased on the multi-panel antenna array's structural properties.Determining in Step 110 of FIG. 2 may for instance involve determining amulti-panel precoding codebook based on the one or more structuralproperties. Indeed, there may be multiple different possible precodingcodebooks that respectively correspond to different possible ways inwhich antenna arrays are structurable into multiple panels. The device16 may need to determine which one of those possible codebooks to usefor the particular array's structural properties. Regardless, thedetermining may then involve selecting the precoder from the determinedmulti-panel precoding codebook, based on measurement of one or morereference signals transmitted from the multi-panel antenna array 14.

In some embodiments, the multi-panel precoding codebook may be aninter-panel precoding codebook (e.g., inter-panel cophasing precodingcodebook). This codebook may be dedicated to precoding across the panelsof the array 14 (e.g., via scalar or vector quantization as describedmore fully below), as opposed to precoding within each individual array.Precoders in the inter-panel precoding codebook may for instance performprecoding so as to co-phase panels of the array (e.g., in order toperform coherent multi-panel transmission whereby antenna portscorresponding to different panels are combined onto the sametransmission layer). This may aim to compensate for phase offsetattributable to the non-uniform and/or uncalibrated nature of thearray's panels.

In some embodiments, for instance, the multiple panels include at leastsome panels that have identical antenna port layouts and intra-panelantenna port indices. In this case, an actual or recommended precoder(that co-phases panels) may apply the same phase offset between antennaports that correspond to spatially adjacent panels and that have thesame intra-panel antenna port indices. That is, considering twoidentical panels and where the antenna ports corresponding to a panel isindexed with i so that (i,0) denotes antenna port number I correspondingto a first panel and (i,1) denotes antenna number I corresponding to asecond panel, then the phase shift between (i,0)->(i,1) is the same forall antenna ports i.

Alternatively or additionally, an actual or recommended precoder (thatco-phases panels) may be configured to apply, for each of the multiplepanels, a panel-specific complex weight to all antenna portscorresponding to that antenna panel. This panel-specific complex weightmay be for instance a phase shift with unitary amplitude.

Regardless, the device 16 may signal the recommended precoder to thetransmit radio node 12 as an index into the inter-panel precodingcodebook. The device 16 in some embodiments may further signal an indexindicating a recommended intra-panel precoder selected from anintra-panel precoding codebook and/or an index indicating a recommendedpolarization co-phasing precoder selected from a polarization co-phasingprecoding codebook.

As explained more fully below, for instance, the overall precodingW_(MP) applied to the transmission 18 may be factorized into W₁, W₂,and/or W₃ precoders. In some embodiments, the recommended precoder is aparticular W₃ precoder from an inter-panel precoding codebook comprisingmultiple different possible W₃ precoders. The device 16 may recommendsuch a precoder by signaling an index (e.g., precoding matrix indicator,PMI) into the W₃ precoder codebook. The codebook may be for instance theLTE 4TX rank-1 Householder codebook, a subsampled and/or puncturedversion of the LTE 4TX rank-1 Householder codebook, or a DFT codebook,as described more fully below.

The device 16 may separately signal an index into a W₁ precoder codebook(i.e., an inter-panel precoding codebook) and/or a W₂ precoder codebook(i.e., a polarization co-phasing precoding codebook).

Alternatively, the recommended precoder may be a precoder that combinesa particular W₃ precoder with a particular W₂ precoder (e.g., resultingin a particular W₂₃ as described below) and/or a particular W₁ precoder.

In any event, tailoring precoding (e.g., in terms of codebook design)for the multi-panel nature of multi-panel antenna arrays may proveadvantageous in that it improves the quality of precoding as well asprecoder recommendations (and CSI reports in general), e.g., as comparedto precoding schemes designed implicitly assuming a single-panel array.This is especially true for Discrete Fourier Transform (DFT) precoding,as applying a DFT precoder across multiple antenna panels may lead topoor precoding performance, especially if the antenna panels areuncalibrated with respect to one another and/or mismatched in terms oftheir antenna element spacing.

Note that some embodiments herein have contemplated feedback of arecommended precoder using codebook-based precoding. Other embodimentsherein extend to non-codebook-based precoding, whereby the CSI feedbackis nonetheless determined based on the one or more structural propertiesof the array 14 in a similar way as described above for codebook-basedprecoding.

Also note that, although FIG. 1 illustrates radio node 12 in the form ofa base station and wireless device 16 in the form of a UE, such need notbe the case. Radio node 12 in other embodiments may be a UE and wirelessdevice 16 may be a base station. Accordingly, embodiments herein may beapplicable for either the uplink or downlink direction. Even further,embodiments herein may also be used for machine-to-machinecommunication, e.g., both nodes 12, 16 are UEs.

Still further, although FIG. 1 illustrates device 16 as transmittingsignaling 20 to the same radio node 12 from which reference signal(s)were received, such need not be the case. In other embodiments, thedevice 16 may transmit signaling to a different radio node.

In general, therefore, embodiments herein are applicable to any type ofwireless communication system 10. Indeed, embodiments may use any of oneor more communication protocols known in the art or that may bedeveloped, such as IEEE 802.xx, CDMA, WCDMA, GSM, LTE, UTRAN, WiMax, orthe like. Accordingly, although sometimes described herein in thecontext of LTE or 5G, the principles and concepts discussed herein areapplicable to other systems as well.

A radio node as used herein is therefore any type node capable ofcommunicating with another radio node wirelessly over radio signals. Aradio node may for example be a radio network node, e.g., in a radioaccess network (RAN) of the system 10. The radio network node may forinstance be a base station, a relay node, or the like.

Alternatively, a radio node may be a wireless device and may thereforerefer to a user equipment (UE), a mobile station, a laptop, asmartphone, a machine-to-machine (M2M) device, a machine-typecommunications (MTC) device, a narrowband Internet of Things (IoT)device, etc. That said, although a radio node in the form of a wirelessdevice may be a UE, it should be noted that the wireless device does notnecessarily have a “user” in the sense of an individual person owningand/or operating the device. A wireless device may also be referred toas a wireless communication device, a radio device, a radiocommunication device, a wireless terminal, or simply a terminal—unlessthe context indicates otherwise, the use of any of these terms isintended to include device-to-device UEs or devices, machine-typedevices or devices capable of machine-to-machine communication, sensorsequipped with a wireless device, wireless-enabled table computers,mobile terminals, smart phones, laptop-embedded equipped (LEE),laptop-mounted equipment (LME), USB dongles, wireless customer-premisesequipment (CPE), etc. In the discussion herein, the termsmachine-to-machine (M2M) device, machine-type communication (MTC)device, wireless sensor, and sensor may also be used. It should beunderstood that these devices may be UEs, but may be generallyconfigured to transmit and/or receive data without direct humaninteraction.

In an IOT scenario, a wireless device as described herein may be, or maybe comprised in, a machine or device that performs monitoring ormeasurements, and transmits the results of such monitoring measurementsto another device or a network. Particular examples of such machines arepower meters, industrial machinery, or home or personal appliances, e.g.refrigerators, televisions, personal wearables such as watches etc. Inother scenarios, a wireless communication device as described herein maybe comprised in a vehicle and may perform monitoring and/or reporting ofthe vehicle's operational status or other functions associated with thevehicle.

In view of the above variations and modifications, a wirelesscommunication device 16 as described above may perform any of theprocessing herein by implementing any functional means or units. In oneembodiment, for example, the wireless communication device 16 comprisesrespective circuits or circuitry configured to perform the steps shownin FIG. 2. The circuits or circuitry in this regard may comprisecircuits dedicated to performing certain functional processing and/orone or more microprocessors in conjunction with memory. In embodimentsthat employ memory, which may comprise one or several types of memorysuch as read-only memory (ROM), random-access memory, cache memory,flash memory devices, optical storage devices, etc., the memory storesprogram code that, when executed by the one or more processors, carriesout the techniques described herein.

FIG. 4 illustrates a wireless communication device 16 in accordance withone or more embodiments. As shown, the wireless communication device 16includes processing circuitry 300 and communication circuitry 310. Thecommunication circuitry 310 (e.g., radio circuitry) is configured totransmit and/or receive information to and/or from one or more othernodes, e.g., via any communication technology. The communicationcircuitry 310 may do so for instance via one or more antennas, which maybe internal or external to the wireless communication device 16. Theprocessing circuitry 300 is configured to perform processing describedabove, e.g., in FIG. 2 such as by executing instructions stored inmemory 320. The processing circuitry 300 in this regard may implementcertain functional means, units, or modules.

FIG. 5 illustrates a wireless communication device 16 in accordance withone or more other embodiments. As shown, the wireless communicationdevice 16 implements various functional means, units, or modules, e.g.,via the processing circuitry 300 in FIG. 4 and/or via software code.These functional means, units, or modules, e.g., for implementing themethod in FIG. 2, include for instance a determining unit or module 410for determining the recommended precoder as described above, and asignaling unit or module 400 for signaling the determined precoder tothe transmit radio node 12.

Note also that a transmit radio node 12 as described above may performany of the processing herein by implementing any functional means orunits. In one embodiment, for example, the transmit radio node 12comprises respective circuits or circuitry configured to perform thesteps shown in any of FIG. 3. The circuits or circuitry in this regardmay comprise circuits dedicated to performing certain functionalprocessing and/or one or more microprocessors in conjunction withmemory. In embodiments that employ memory, which may comprise one orseveral types of memory such as read-only memory (ROM), random-accessmemory, cache memory, flash memory devices, optical storage devices,etc., the memory stores program code that, when executed by the one ormore processors, carries out the techniques described herein.

FIG. 6 illustrates a transmit radio node 12 in accordance with one ormore embodiments. As shown, the transmit radio node 12 includesprocessing circuitry 500 and communication circuitry 510. Thecommunication circuitry 510 (e.g., radio circuitry) is configured totransmit and/or receive information to and/or from one or more othernodes, e.g., via any communication technology. The communicationcircuitry 510 may do so for instance via the multi-panel antenna array14, which may be internal or external to the transmit radio node 12. Theprocessing circuitry 500 is configured to perform processing describedabove, e.g., in FIG. 3, such as by executing instructions stored inmemory 520. The processing circuitry 500 in this regard may implementcertain functional means, units, or modules.

FIG. 7 illustrates a transmit radio node 12 in accordance with one ormore other embodiments. As shown, the transmit radio node 12 implementsvarious functional means, units, or modules, e.g., via the processingcircuitry 500 in FIG. 6 and/or via software code. These functionalmeans, units, or modules, e.g., for implementing the method in FIG. 3,include for instance a transmitting and/or receiving unit or module 600for transmitting signaling 22, receiving signaling 20, and/ortransmitting the transmission 18. Also included may be a precoding unitor module 620 for determining the precoder to apply and/or for precodingthe transmission 18.

Those skilled in the art will also appreciate that embodiments hereinfurther include corresponding computer programs.

A computer program comprises instructions which, when executed on atleast one processor of a node 12 or 16, cause the node 12 or 16, tocarry out any of the respective processing described above. A computerprogram in this regard may comprise one or more code modulescorresponding to the means or units described above.

Embodiments further include a carrier containing such a computerprogram. This carrier may comprise one of an electronic signal, opticalsignal, radio signal, or computer readable storage medium.

Without loss of generality, one or more embodiments herein will now bedescribed, sometimes with reference to New Radio (NR) or 5G terminology.For example, some embodiments are described with reference to a NextGeneration NodeB (gNB) and/or a user equipment (UE). However, theseembodiments are not limited to NR or 5G technology, but are to beextended more generally to any wireless technology (e.g., LTE or 4G andbeyond). In this regard, aspects described with regard to an gNB and/orUE below may be attributed to the more general terminology used above;namely, a radio network node (of which a gNB is an example) and awireless communication device (of which a UE is an example).

Some embodiments herein comprise methods for channel state information(CSI) feedback that enable multiple-input multiple-output (MIMO)transmission from multi-panel antenna arrays, and more specifically,comprise precoder codebook designs that enable coherent and/ornon-coherent multi-panel transmission. Known approaches design precodercodebooks for CSI feedback with single-panel antenna arrays in mind.Directly applying such CSI feedback to systems where multi-panel antennaarrays are used may lead to poor performance since the structure of themulti-panel antenna array is not taken into account in the CSI feedback.Better system throughput can thus be achieved if the CSI feedbackreflects the multi-panel antenna array (e.g., its structure). This maybe attained by using a precoder codebook structure that reflects themulti-panel antenna array being used by the transmitter, and the designof such a multi-panel precoder codebook structure is the object of someembodiments herein.

In some embodiments, a wireless communication device (e.g., a userequipment, UE) is configured to report CSI feedback corresponding to thecertain multi-panel antenna array setup used at the Next Generation NodeB (gNB). The configuration comprises signaling of codebook parametersenabling the UE to determine the precoder codebook to use forcalculating CSI feedback. Comprised in the codebook parameters areparameters relating to the multi-panel setup along with other parameterssuch as the number of antenna ports in the codebook. In someembodiments, the multi-panel codebook is designed with a two-dimensionalpanel setup in mind, and the number of antenna panels in each dimension,signaled to the UE. In other embodiments, the spatial distribution ofthe antenna panels does not need to be known in order to determine thecodebook and only the number of antenna panels is signaled to the UE.

The signaling of codebook parameters may be done via radio resourcecontrol (RRC) signaling, and may be part of a configuration of a CSIprocess or a configuration of a channel state information referencesignal (CSI-RS) resource, for instance. Such RRC configuration istypically done in a semi-static fashion, so that the codebook isconfigured once when connecting to the cell or serving node and is notexpected to be changed that often. The codebook parameters may also besignaled in a more dynamic fashion. For instance, the signaling can becomprised in a control information message, such as a Downlink Controlinformation (DCI) message transmitted on a Physical Downlink ControlChannel (PDCCH). As one example, the DCI could include an Uplink Grantfor transmission of a CSI feedback report, where the codebook to use forcalculating the CSI report is indicated in the Uplink Grant.

In some cases, a set of codebook parameters may be signaled to the UE ina semi-static fashion, such as via RRC signaling, and the selection ofwhich one of the set of codebook parameters to use may be signaled in amore dynamic fashion. Further, the codebook parameters may also besignaled in a medium access control (MAC) Control Element (MAC CE) or ina MAC header. Anyhow, regardless of how the codebook parameters aresignaled, the UE is able to determine which codebook to use based on thecodebook parameters.

The UE is then explicitly or implicitly instructed to report CSIfeedback using the determined codebook. The UE may, for instance, beconfigured with periodic CSI reporting, meaning that a CSI report shouldbe transmitted periodically with a fixed periodicity. The UE may also,for instance, be configured with aperiodic CSI reporting, meaning thatCSI is only reported when the gNB sends a CSI request in DCI. Anyhow, inorder to report CSI feedback, the gNB must first transmit a set ofCSI-RS from the multiple antenna panels, where each CSI-RS in the set istransmitted from an antenna port. In some embodiments, the set of CSI-RSbelong to the same CSI-RS resource, while in other embodiments, CSI-RStransmitted from different antenna panels belong to different CSI-RSresources. In some further embodiments, the several CSI-RS resources istransmitted per panel and the UE first performs a selection of thepreferred CSI-RS resource for each panel (for instance by feeding back aCSI-RS Resource Indication (CRI) for each panel).

Based on the CSI-RS measurements, the UE can select one or more precodermatrix from the determined precoder codebook that results in the bestachievable throughput, and transmit one or more precoder matrixindicators, PMIs, back to the gNB.

In more detail, multi-antenna techniques can significantly increase thedata rates and reliability of a wireless communication system. Theperformance is in particular improved if both the transmitter and thereceiver are equipped with multiple antennas, which results in amultiple-input multiple-output (MIMO) communication channel. Suchsystems and/or related techniques are commonly referred to as MIMO.

The NR standard is currently being specified. A core component in NR isthe support of MIMO antenna deployments and MIMO related techniques. Itis expected that NR will support up to 8- or 16-layer spatialmultiplexing for up to 32 or 64 antenna ports with channel dependentprecoding. The spatial multiplexing mode is aimed for high data rates infavorable channel conditions. An illustration of the spatialmultiplexing operation is provided in FIG. 8.

As seen, the information carrying symbol vector s is multiplied by anNT×r precoder matrix W, which serves to distribute the transmit energyin a subspace of the NT (corresponding to NT antenna ports) dimensionalvector space. The precoder matrix is typically selected from a codebookof possible precoder matrices, and typically indicated by means of aprecoder matrix indicator (PMI), which specifies a unique precodermatrix in the codebook for a given number of symbol streams. The rsymbols in s each correspond to a layer and r is referred to as thetransmission rank. In this way, spatial multiplexing is achieved sincemultiple symbols can be transmitted simultaneously over the sametime/frequency resource element (TFRE). The number of symbols r istypically adapted to suit the current channel properties.

NR uses OFDM in the downlink (and OFDM or DFT precoded OFDM in theuplink) and hence the received NR×1 vector yn for a certain TFRE onsubcarrier n (or alternatively data TFRE number n) is thus modeled by

y _(n) =H _(n) Ws _(n) +e _(n)  Equation 1

where e_(n) is a noise/interference vector obtained as realizations of arandom process. The precoder W can be a wideband precoder, which isconstant over frequency, or frequency selective.

The precoder matrix W is often chosen to match the characteristics ofthe NR×NT MIMO channel matrix H_(n), resulting in so-called channeldependent precoding. This is also commonly referred to as closed-loopprecoding and essentially strives for focusing the transmit energy intoa subspace which is strong in the sense of conveying much of thetransmitted energy to the UE. In addition, the precoder matrix (orsimply “precoder”) may also be selected to strive for orthogonalizingthe channel, meaning that after proper linear equalization at the UE,the inter-layer interference is reduced.

One example method for a UE to select a precoder matrix W can be toselect the W_(k) that maximizes the Frobenius norm of the hypothesizedequivalent channel:

$\begin{matrix}{\max\limits_{k}{{{\overset{\hat{}}{H}}_{n}W_{k}}}_{F}^{2}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

where

-   -   Ĥ_(n) is a channel estimate, possibly derived from CSI-RS as        described in Section 0 below.    -   W_(k) is a hypothesized precoder matrix with index k.    -   Ĥ_(n)W_(k) is the hypothesized equivalent channel

In closed-loop precoding for the NR downlink, the UE transmits, based onchannel measurements in the forward link (downlink), recommendations tothe gNodeB of a suitable precoder to use. The gNodeB configures the UEto provide feedback according to the UE's transmission mode, and maytransmit CSI-RS and configure the UE to use measurements of CSI-RS tofeed back recommended precoding matrices that the UE selects from acodebook. A single precoder that is supposed to cover a large bandwidth(wideband precoding) may be fed back. It may also be beneficial to matchthe frequency variations of the channel and instead feed back afrequency-selective precoding report, e.g. several precoders, one persubband. This is an example of the more general case of channel stateinformation (CSI) feedback, which also encompasses feeding back otherinformation than recommended precoders to assist the eNodeB insubsequent transmissions to the UE. Such other information may includechannel quality indicators (CQIs) as well as transmission rank indicator(RI).

Given the CSI feedback from the UE, the gNodeB determines thetransmission parameters it wishes to use to transmit to the UE,including the precoding matrix, transmission rank, and modulation andcoding state (MCS). These transmission parameters may differ from therecommendations the UE makes. Therefore a rank indicator and MCS may besignaled in downlink control information (DCI), and the precoding matrixcan be signaled in DCI or the gNodeB can transmit a demodulationreference signal from which the equivalent channel can be measured. Thetransmission rank, and thus the number of spatially multiplexed layers,is reflected in the number of columns of the precoder W. For efficientperformance, it is important that a transmission rank that matches thechannel properties is selected.

Channel State Information Reference Symbols (CSI-RS)

In NR, there exists a reference symbol sequence for the intent toestimate downlink channel state information, the CSI-RS. By measuring aCSI-RS transmitted from the gNodeB, a UE can estimate the effectivechannel the CSI-RS is traversing including the radio propagation channeland antenna gains. In more mathematical rigor this implies that if aknown CSI-RS signal X is transmitted, a UE can estimate the couplingbetween the transmitted signal and the received signal (i.e., theeffective channel). Hence if no virtualization is performed in thetransmission, the received signal y can be expressed as

y=Hx+e  Equation 3

and the UE can estimate the effective channel H.

Up to 32 or 64 CSI-RS ports can be configured in NR, that is, the UE canestimate the channel from up to 32 or 64 transmit antenna ports.

Based on a specified CSI-RS resource and on an interference measurementconfiguration (e.g. a zero-power CSI-RS resource), the UE can estimatethe effective channel and noise plus interference, and consequently alsodetermine the rank, precoding matrix, and MCS to recommend to best matchthe particular channel.

2D (Single-Panel) Antenna Arrays

Multi-antenna transmission in NR is generally envisioned to be used withtwo dimensional antenna arrays. Such antenna arrays may be (partly)described by the number of antenna columns corresponding to thehorizontal dimension N, the number of antenna rows corresponding to thevertical dimension M and the number of dimensions corresponding todifferent polarizations P, with e.g. P=2 for dual polarized antennaelements (e.g. cross-polarized antenna elements). The total number ofantennas is thus N_(antennas)=NMP. It should be pointed out that theconcept of an antenna is non-limiting in the sense that it can refer toany virtualization (e.g., linear mapping) of the physical antennaelements. For example, pairs of physical sub-elements could be fed thesame signal, and hence share the same virtualized antenna port.

In FIG. 9, an N×M array with N horizontal antenna elements 900 and Mvertical antenna elements 900 is shown for an example of 4×4cross-polarized (P=2) antenna elements.

Precoding may be interpreted as multiplying the signal with differentbeamforming weights for each antenna prior to transmission. A typicalapproach is to tailor the precoder to the antenna form factor, i.e.taking into account N, M and P when designing the precoder codebook.

Multi-Panel Antenna Arrays

When building very large antenna arrays, it can be challenging to fit inall the hardware components into a single antenna panel. One buildingpractice is to use a modular approach and construct a, so called,multi-panel antenna array consisting of multiple antenna panels (asdefined in the previous section). In the general case, the spacingbetween the right-most antenna element of a first panel and theleft-most antenna element of a second panel placed to the right of thefirst panel can be larger than the spacing between antenna elementswithin a panel, corresponding to a non-uniform multi-panel array. It isgenerally assumed that the tight calibration required for seamlesscoherent transmission between antenna elements is only done within eachpanel, and so, different panels of the multi-panel array can beuncalibrated. There may thus exist a frequency offset, timingmisalignment, and a LO phase offset between the panels.

A multi-panel array can, for example, be parametrized in the number ofvertical panels M_(g), the number of horizontal panels N_(g) and size ofthe constituent panels M, N, P. An example of a multi-panel antennaarray with Mg=2, Ng=2 and P=2, thus resulting in 4 panels 00, 01, 10 and11 is given in FIG. 10. Each panel may be an N×M array, e.g. a 4×4 arraywith cross polarized antenna elements as shown in FIG. 9.

DFT-Based Precoders

A common type of precoding is to use a DFT precoder, where the precodervector used to precode a single-layer transmission using asingle-polarized uniform linear array (ULA) with N₁ antennas is definedas

$\begin{matrix}{{w_{1D}\left( {l,N_{1},O_{1}} \right)} = {\frac{1}{\sqrt{N_{1}}}\begin{bmatrix}e^{j\; 2\;{\pi \cdot 0 \cdot \frac{l}{O_{1}N_{1}}}} \\e^{j\; 2\;{\pi \cdot 1 \cdot \frac{l}{O_{1}N_{1}}}} \\\vdots \\e^{j\; 2\;{\pi \cdot {({N_{1} - 1})}}\frac{l}{O_{1}N_{1}}}\end{bmatrix}}} & {{Equation}\mspace{14mu} 4}\end{matrix}$

where 1=0,1, . . . O₁N₁−1 is the precoder index and O₁ is an integeroversampling factor. A precoder for a dual-polarized uniform lineararray (ULA) with N₁ antennas per polarization (and so 2N₁ antennas intotal) can be similarly defined as

$\begin{matrix}{{w_{{1D},{DP}}\left( {l,{N_{1,}O_{1}}} \right)} = {\begin{bmatrix}{w_{1D}(l)} \\{e^{j\phi}{w_{1D}(l)}}\end{bmatrix} = {\begin{bmatrix}{w_{1D}(l)} & 0 \\0 & {w_{1D}(l)}\end{bmatrix}\begin{bmatrix}1 \\e^{j\phi}\end{bmatrix}}}} & {{Equation}\mspace{14mu} 5}\end{matrix}$

where e^(jϕ) is a co-phasing factor between the two polarizations thatmay for instance be selected from a QPSK alphabet

$\phi \in {\left\{ {0,\frac{\pi}{2},\pi,\frac{3\pi}{2}} \right\}.}$

A corresponding precoder vector for a two-dimensional uniform planararrays (UPA) with N₁×N₂ antennas can be created by taking the Kroneckerproduct of two precoder vectors asw_(2D)(l,m)=w_(1D)(l,N₁,O₁)⊗w_(1D)(m,N₂,O₂), where O₂ is an integeroversampling factor in the N₂ dimension. Each precoder w_(2D)(l,m) formsa DFT beam, all the precoders {w_(2D)(l,m), l=0, . . . , N₁O₁−1; m=0, .. . , N₂O₂−1} form a grid of DFT beams. An example is shown in FIG. 11,wherein (N₁,N₂)=(4,2), O1=4 and O2=4, resulting in a (N₁O₁,N₂O₂)=(16,8)grid with 16 beams in a first or “I-dimension” and 8 beams in a secondor “m-dimension”. Each beam thus may by be characterized by a pair ofdimension parameters I and m. Beams with (I.m)=(0,0), (4,0), (8,0),(12,0), (0,4), (4,4), (8,4), (12,4) form orthogonal DFT beams 110,whereas all the other beams form oversampled beams 111. By way ofexample, DFT beam 112 with l=2, m=1 corresponds to precoderw_(2D)(l=2,m=1).

Throughout the following sections, the terms DFT beams' and DFTprecoders' are used interchangeably.

More generally, a beam with an index pair (l,m) can be identified by thedirection in which the greatest energy is transmitted when precodingweights w_(2D)(l,m) are used in the transmission. Also, a magnitudetaper can be used with DFT beams to lower the beam's sidelobes. A 1D DFTprecoder along N₁ and N₂ dimensions with magnitude tapering can beexpressed as

${{w_{1D}\left( {l,N_{1},O_{1},\beta} \right)} = {\frac{1}{\sqrt{N_{1}}}\begin{bmatrix}{\beta_{0}e^{j\; 2\;{\pi \cdot 0 \cdot \frac{l}{O_{1}N_{1}}}}} \\{\beta_{1}e^{j\; 2\;{\pi \cdot 1 \cdot \frac{l}{O_{1}N_{1}}}}} \\\vdots \\{\beta_{N_{1} - 1}e^{j\; 2\;{\pi \cdot {({N_{1} - 1})}}\frac{l}{O_{1}N_{1}}}}\end{bmatrix}}},{{w_{1D}\left( {m,N_{2},O_{2},\gamma} \right)} = {\frac{1}{\sqrt{N_{2}}}\begin{bmatrix}{\gamma_{0}e^{j\; 2{\pi \cdot 0 \cdot \frac{m}{O_{2}N_{2}}}}} \\{\gamma_{0}e^{j\; 2{\pi \cdot 1 \cdot \frac{m}{O_{2}N_{2}}}}} \\\vdots \\{\gamma_{N_{2} - 1}e^{j\; 2{\pi \cdot {({N_{2} - 1})} \cdot \frac{m}{O_{2}N_{2}}}}}\end{bmatrix}}}$

Where 0<β_(i),γ_(k)≤1 (i=0,1, . . . , N₁−1; k=0,1, . . . , N₂−1) areamplitude scaling factors. β_(i)=1, γ_(k)=1 (i=0,1, . . . , N₁−1; k=0,1,. . . , N₂−1) correspond to no tapering. DFT beams (with or without amagnitude taper) have a linear phase shift between elements along eachof the two dimensions. Without loss of generality, we assume that theelements of w(l,m) are ordered according tow(l,m)=w_(1D)(1,N₁,O₁,β)⊗w_(1D)(m,N₂,O₂,γ) such that adjacent elementscorrespond to adjacent antenna elements along dimension N₂, and elementsof w(l,m) spaced N₂ apart correspond to adjacent antenna elements alongdimension N₁. Then the phase shift between two elements w_(s) ₁ (l,m)and w_(s) ₂ (l,m) of w(l,m) can be expressed as:

${w_{s_{2}}\left( {l,m} \right)} = {{w_{s_{1}}\left( {l,m} \right)} \cdot \left( \frac{\alpha_{s_{2}}}{\alpha_{s_{1}}} \right) \cdot e^{j2{\pi{({{{({k_{1} - i_{1}})}\Delta_{1}} + {{({k_{2} - i_{2}})}\Delta_{2}}})}}}}$

Where

-   -   s₁=i₁N₂+i₂ and s₂=k₁N₂+k₂ (with 0≤i₂<N₂, 0≤i₁<N₁, 0≤k₂<N₂, and        0≤k₁<N₁) are integers identifying two entries of the beam w(l,m)        so that (i₁,i₂) indicates to a first entry of beam w(l,m) that        is mapped to a first antenna element (or port) and (k₁,k₂)        indicates to a second entry of beam w(l,m) that is mapped to a        second antenna element (or port).    -   α_(s) ₁ =β_(i) ₁ γ_(i) ₂ and α_(s) ₂ =β_(k) ₁ γ_(k) ₂ are real        numbers. α_(i)≠1 (i=s₁,s₂) if magnitude tapering is used;        otherwise α_(i)=1.

$\Delta_{1} = \frac{l}{O_{1}N_{1}}$

is a phase shift corresponding to a direction along an axis, e.g. thehorizontal axis (‘azimuth’)

$\Delta_{2} = \frac{m}{O_{2}N_{2}}$

is a phase shift corresponding to direction along an axis, e.g. thevertical axis (‘elevation’)

Therefore a kth beam d(k) formed with precoder w(l_(k),m_(k)) can alsobe referred to by the corresponding precoder w(l_(k),m_(k)), i.e.d(k)=w(l_(k),m_(k)). Thus a beam d(k) can be described as a set ofcomplex numbers, each element of the set being characterized by at leastone complex phase shift such that an element of the beam is related toany other element of the beam where d(k)=d_(i)(k)α_(i,n)e^(j2π(pΔ)^(1,k) ^(+qΔ) ^(2,k) ⁾=d_(i)(k)α_(i,n)(e^(j2πΔ) ^(1,k) )^(p)(ee^(j2πΔ)^(2,k) )^(q), where d_(i)(k) is the ith element of a beam d(k), α_(i,n)is a real number corresponding to the ith and nth elements of the beamd(k); p and q are integers; and Δ_(1,k) and Δ_(2,k) are real numberscorresponding to a beam with index pair (l_(k),m_(k)) that determine thecomplex phase shifts e^(j2πΔ) ^(1,k) and e^(j2πΔ) ^(2,k) , respectively.Index pair (l_(k),m_(k)) corresponds to a direction of arrival ordeparture of a plane wave when beam d(k) is used for transmission orreception in a UPA or ULA. A beam d(k) can be identified with a singleindex k where=l_(k)+N₁O₁m_(k), i.e, along vertical or N₂ dimensionfirst, or alternatively k=N₂O₂l_(k)+m_(k), i.e. along horizontal or N₁dimension first.

Extending the precoder for a dual-polarized ULA may then be done as

$\begin{matrix}{{W_{{2D},{DP}}\left( {l,m,\phi} \right)} = {{\begin{bmatrix}1 \\e^{j\phi}\end{bmatrix} \otimes {w_{2D}\left( {l,m} \right)}} = \mspace{20mu}{\left\lbrack \begin{matrix}{w_{2D}\left( {l,m} \right)} \\{e^{j\phi}{w_{2D}\left( {l,m} \right)}}\end{matrix} \right\rbrack = {\left\lbrack \begin{matrix}{w_{2D}\left( {l,m} \right)} & 0 \\0 & {w_{2D}\left( {l,m} \right)}\end{matrix} \right\rbrack\begin{bmatrix}1 \\e^{j\phi}\end{bmatrix}}}}} & {{Equation}\mspace{14mu} 6}\end{matrix}$

A precoder matrix W_(2D,DP) for multi-layer transmission may be createdby appending columns of DFT precoder vectors as

W _(2D,DP) ^((R))=[w _(2D,DP)(l ₁ ,m ₁,ϕ₁)w _(2D,DP)(l ₂ ,m ₂,ϕ₂) . . .w _(2D,DP)(l _(R) ,m _(R),ϕ_(R))]

where R is the number of transmission layers, i.e. the transmissionrank. In a special case for a rank-2 DFT precoder, m₁=m₂=m and l₁=l₂=1,we have

$\begin{matrix}{{W_{{2D},{DP}}^{(2)}\left( {l,m,\phi_{1},\phi_{2}} \right)} = {\begin{matrix}\left\lbrack {w_{{2D},{DP}}\left( {l,m,\phi_{1}} \right)} \right. & \left. {w_{{2D},{DP}}\left( {l,m,\phi_{2}} \right)} \right\rbrack\end{matrix} = {\left\lbrack \begin{matrix}{w_{2D}\left( {l,m} \right)} & 0 \\0 & {w_{2D}\left( {l,m} \right)}\end{matrix} \right\rbrack\left\lbrack \begin{matrix}1 & 1 \\e^{j\phi_{1}} & e^{j\phi_{2}}\end{matrix} \right\rbrack}}} & {{Equation}\mspace{14mu} 7}\end{matrix}$

For each rank, all the precoder candidates form a precoder ‘codebook’ ora ‘codebook’. A UE can first determine the rank of the estimateddownlink wideband channel based CSI-RS. After the rank is identified,for each subband the UE then searches through all the precodercandidates in a codebook for the determined rank to find the bestprecoder for the subband. For example, in case of rank=1, the UE wouldsearch through w_(2D,DP)(k,l,ϕ) for all the possible (k,l,ϕ) values. Incase of rank=2, the UE would search through W_(2D,DP) ⁽²⁾(k,l,ϕ₁,ϕ₂) forall the possible (k,l,ϕ₁,ϕ₂) values.

Typically, the required frequency granularity of the selection of theDFT beam direction and the polarization co-phasing are different, theDFT beam can typically be selected once for the entire bandwidth whilethe polarization co-phasing can benefit from per-subband selection. Atypical approach is thus to split up the DFT beam selection andpolarization co-phasing in separate matrix factors as below:

$\begin{matrix}{{W_{{2D},{DP}}^{(2)}\left( {l,m,\ \phi_{1},\phi_{2}} \right)} = {{\begin{bmatrix}{w_{2D}\left( {l,m} \right)} & 0 \\0 & {w_{2D}\left( {l,m} \right)}\end{bmatrix}\begin{bmatrix}1 & 1 \\e^{j\phi_{1}} & e^{j\phi_{2}}\end{bmatrix}} = {W_{1}W_{2}}}} & {{Equation}\mspace{14mu} 8}\end{matrix}$

The W₁ matrix factor can then be reported once for the entire bandwidthwhile the W₂ matrix factor can be reported on a subband basis.

Notation, Assumptions, and Scope

For ease of explanation in the following, it will be assumed thatM_(g)=N_(g)=2, resulting in 4 antenna panels 1010, 1020, 1030 and 1040,as is illustrated in FIG. 10. It should be noted that some embodimentsare applicable to any number of antenna panels larger than one, and thatthe number and spatial distribution of panels used in the embodimentsherein is merely one example.

Assume also in the following that the antenna elements of the panelarray are indexed with

i=M _(g)PNM·r _(V)+PNM·r _(H) +NM·p+M·n+m,

where r_(V)=0, . . . , M_(g)−1 is the vertical panel index, r_(H)=0, . .. , N_(g)−1 is the horizontal panel index, m=0, . . . , M−1 is thevertical antenna port index within a panel, n=0, . . . , N−1 is thehorizontal antenna port index within a panel and p=0,1 is thepolarization index. That is, the multi-panel precoding matrix W_(MP) isconstructed by stacking the precoding matrices for the constituentpanels W_(P00), W_(P10), W_(P01), W_(P11) so that

$W_{MP} = {\begin{bmatrix}W_{P00} \\W_{P10} \\W_{P01} \\W_{P11}\end{bmatrix}.}$

Note that this is just one example of how the antenna ports may beordered. Different embodiments may of course utilize a different antennaordering.

In the following, the Kronecker product will be used. The Kroneckerproduct A⊗B between two matrices

$A = \begin{bmatrix}A_{1,1} & \cdots & A_{1,M} \\\vdots & \ddots & \vdots \\A_{N,1} & \cdots & A_{N,M}\end{bmatrix}$

and B is defined as

${{A \otimes B} = \begin{bmatrix}{A_{1,1}B} & \cdots & {A_{1,M}B} \\\vdots & \ddots & \vdots \\{A_{N,1}B} & \cdots & {A_{N,M}B}\end{bmatrix}},$

i.e. the matrix B is multiplied to each element of A. The Kroneckerproduct rule (A⊗B)(C⊗D)=(AC)⊗(BD) is a useful mathematical property forrewriting matrix expressions which is used extensively hereinafter.Further, the notation I_(K) will denote a size K×K identity matrix.

Motivation for Design of Codebook for Coherent Multi-Panel Transmission

A DFT precoder codebook, such as that described in the backgroundsection, comprises precoder vectors with linearly increasing phases overthe antenna ports in each spatial dimension. Such a codebook designimplicitly assumes an antenna setup of phase-calibrated and equallyspaced antenna ports in each dimension. In this case, the codebookperfectly matches the array response assuming a pure line-of-sightchannel and gives a good representation of the dominant channel path forother propagation conditions. In the case of an uncalibrated multi-panelarray, and/or, a non-uniform multi-panel array, the implicit assumptionsof the DFT codebook are thus broken. That is, applying a DFT precoderacross antenna elements of the multiple panels may not result in anefficient representation of the channel response. This is due to severalfactors.

First, the spacing between the last antenna element of a panel and thefirst antenna element of the next panel is different from the antennaelement spacing within a panel for a non-uniform panel array. Thus, thephase shift between said antenna elements would have to be

$e^{\frac{j2\pi{k{({1 + \Delta_{pa{nel}}})}}}{N}}$

rather than

$e^{\frac{j2\pi k}{N}}$

(as it is for the DFT precoder) in order to create a linear phase front,where k is the DFT precoder index, N the number of antennas in adimension and Δ_(panel) the additional distance between panels comparedto the distance between panels in a uniform multi-panel array. Thisphase difference could of course be compensated directly for in thecodebook if the panel distance was known, however, the distance betweenpanels is up to implementation and not known in the general case.

Second, there may exist an additional phase offset between panels dueto, for instance, different LO phase state. In the worst case, the phaseoffset may be completely random and thus uniformly distributed in [0,2π].

Third, if the antenna panels have a timing misalignment, this mayintroduce a frequency-selective phase offset.

A codebook for coherent multi-panel transmission should thus compensatefor these phase offsets.

Codebook Design for Coherent Multi-Panel Transmission

In this section, codebook designs for coherent multi-panel transmissionare presented. The intent of the codebook design is to compensate forthe phase offsets discussed in the previous section. Generally speaking,the inter-panel phase offset compensation may be described by alength-M_(g)N_(g) vector

${w_{PANEL} = \begin{bmatrix}1 \\e^{j\alpha_{10}} \\e^{j\alpha_{01}} \\e^{j\alpha_{11}}\end{bmatrix}},$

where

e^(jα) ^(r) _(V) ^(r) _(H)

is the phase compensation for panel (r_(V),r_(H)). The phasecompensation factors for an antenna panel should be applied to all theantenna ports corresponding to that panel.

For the precoding within the antenna panel, a DFT type precoder may beused, e.g.

${\begin{bmatrix}{w_{2D}\left( {l,m} \right)} & 0 \\0 & {w_{2D}\left( {l,m} \right)}\end{bmatrix}\begin{bmatrix}1 \\e^{j\phi}\end{bmatrix}} = {W_{1}W_{2}}$

for a rank-1 precoder matrix.

In some embodiments, the antenna panels are co-located, and so, thepropagation characteristics between the UE and each antenna panel may beassumed to be similar, and thus, the same selection of per-panelprecoder can be assumed to be optimal. In that case, the multi-panelprecoder matrix may be expressed as

$W_{MP} = {{w_{PANEL} \otimes \left( {W_{1}W_{2}} \right)} = \begin{bmatrix}{W_{1}W_{2}} \\{e^{j\alpha_{10}}W_{1}W_{2}} \\{e^{j\alpha_{01}}W_{1}W_{2}} \\{e^{j\alpha_{11}}W_{1}W_{2}}\end{bmatrix}}$

where ⊗ denotes the Kronecker product between to matrices. As seen, thisprecoder structure applies the per-panel precoder W₁W₂ to each of thefour (in this example) antenna panels and applies a phase scaling

e^(jα) ^(r) _(V) ^(R) _(H)

to all the antenna ports of panel (r_(V),r_(H)), as was desired. Theexpression can be mathematically rewritten into a series of matrixmultiplications instead of a Kronecker product as

$W_{MP} = {{w_{PANEL} \otimes \left( {W_{1}W_{2}} \right)} = {{\underset{\underset{\overset{\Delta}{=}W_{3}}{︸}}{\left( {w_{{PANE}L} \otimes I_{2MN}} \right)}W_{1}W_{2}} = {W_{3}W_{1}{W_{2}.}}}}$

This form is more convenient to work with. The resulting precoder matrixcan thus be factorized into three matrix factors:

-   -   W₃: Comprises co-phasing of the antenna panels    -   W₁: Comprises DFT precoding within each antenna panel    -   W₂: Comprises polarization co-phasing within each panel

The feedback of the selection of each matrix factor may comprisetransmitting a separate PMI for each matrix factor. It may also comprisejointly encoding the selection of all matrix factors into a single PMI.In some embodiments, several precoder matrices will be selected. Forinstance, a separate selection may be done for each subband in thefrequency domain. In that case, several PMIs indicating all the selectedmatrices may be comprised a CSI feedback report.

In some embodiments, the W₃ matrix factor is selected with a widebandfrequency-granularity. This is applicable when the antennas are fairlywell-calibrated so that no timing misalignment exists between panels. Inother embodiments, the W₃ matrix factor is instead selected with asubband frequency granularity. This is appropriate if there exists atiming misalignment between antenna panels that needs to be compensatedfor.

In some embodiments, the antenna panels may be physically separated fromeach-other or rotated in different directions. In that case, it may notbe beneficial to use the same per-panel precoder. Instead, a separateper-panel precoder(s) W₁ ⁰⁰W₂ ⁰⁰,W₁ ¹⁰W₂ ¹⁰, . . . are used for eachpanel, so that the multi-panel precoder may be expressed as

$W_{MP} = {\begin{bmatrix}{W_{1}^{00}W_{2}^{00}} \\{e^{j\alpha_{10}}W_{1}^{10}W_{2}^{10}} \\{e^{j\alpha_{01}}W_{1}^{01}W_{2}^{01}} \\{e^{j\alpha_{11}}W_{1}^{11}W_{2}^{11}}\end{bmatrix}.}$

Changing Order of the Matrix Factors

It is noted that one may rewrite the matrix expression in order tochange the order of the “inter-panel co-phasing matrix factor”,“per-panel DFT precoding matrix factor” and “polarization co-phasingmatrix factor”. Embodiments herein contemplate any such rewriting of thematrix factor order. Some example embodiments are given below.

In some embodiments, the precoder matrix expression is rewritten to havethe per-panel DFT precoding matrix factor as the first matrix, such as:

${W_{MP} = {{w_{PANEL} \otimes \left( {W_{1}W_{2}} \right)} = {{w_{PANEL} \otimes \left( {\begin{bmatrix}w_{2D} & 0 \\0 & w_{2D}\end{bmatrix}W_{2}} \right)} = {{w_{PANEL} \otimes \left( {\left( {I_{2} \otimes w_{2D}} \right)W_{2}} \right)} = {{w_{PANEL} \otimes \left( {\left( {I_{2} \otimes w_{2D}} \right)\left( {W_{2} \otimes 1} \right)} \right)} = {{w_{PANEL} \otimes \left( {W_{2} \otimes w_{2D}} \right)} = {{\left( {w_{PANEL} \otimes W_{2}} \right) \otimes w_{2D}} = {{\left( {I_{2N_{g}M_{g}} \cdot \left( {w_{PANEL} \otimes W_{2}} \right)} \right) \otimes \left( {w_{2D} \cdot 1} \right)} = {{\left( {I_{2N_{g}M_{g}} \otimes w_{2D}} \right) \cdot \left( {w_{PANEL} \otimes W_{2}} \right)} = {\cdot \left( {w_{PANEL} \otimes W_{2}} \right)}}}}}}}}}},$

where

=I_(2N) _(g) _(M) _(g) ⊗w_(2D) is a per-panel DFT precoding matrixfactor.

In one such embodiment, the expression is rewritten so that theinter-panel co-phasing matrix factor is the middle matrix factor as

W_(MP) = ⋅(w_(PANEL) ⊗ W₂) = ⋅( (w_(PANEL) ⋅ 1) ⊗ (I₂ ⋅ W₂)) = (w_(PANEL) ⊗ I₂)W₂ = W₂,

where

=w_(PANEL)⊗I₂ is an inter-panel co-phasing matrix factor.

In another such embodiment, the inter-panel co-phasing matrix factor isinstead the last matrix factor as

W_(MP) = ⋅(w_(PANEL) ⊗ W₂) = ⋅( (I_(N_(g)M_(g)) ⋅ w_(PANEL)) ⊗ (W₂  ⋅ I_(R))) = (I_(N_(g)M_(g)) ⊗ W₂)(w_(PANEL) ⊗ I_(R)) = ,

where

is a per-panel polarization co-phasing matrix factor and {tilde over({tilde over (W)})}₃ is an inter-panel co-phasing matrix factor and R isthe precoder rank.

A person skilled in the art may appreciate that the disclosed techniqueof rewriting the precoder expression so that the order between thedifferent types of matrix factors is exchanged can be applied to createan arbitrary order of the matrix factors, and so, the embodiments hereinare applicable to any ordering of said matrix factors.

In yet another embodiment, the

W _(MP)=

(w _(PANEL) ⊗W ₂)=

where

=(w_(PANEL)⊗W₂) is a combined inter-panel and per-panel polarizationco-phasing matrix factor. For instance, such a combined matrix may havethe structure:

$= {\left( {w_{PANEL} \otimes W_{2}} \right) = \begin{bmatrix}1 & 1 \\e^{j\phi} & {- e^{j\phi}} \\e^{j\alpha_{10}} & e^{j\alpha_{10}} \\{e^{j\alpha_{10}}e^{j\phi}} & {{- e^{j\alpha_{10}}}e^{j\phi}} \\e^{j\alpha_{01}} & e^{j\alpha_{01}} \\{e^{j\alpha_{01}}e^{j\phi}} & {{- e^{j\alpha_{01}}}e^{j\phi}} \\e^{j\alpha_{11}} & e^{j\alpha_{11}} \\{e^{j\alpha_{11}}e^{j\phi}} & {{- e^{j\alpha_{11}}}e^{j\phi}}\end{bmatrix}}$

Such a combination of the two matrix types may be beneficial if {tildeover (W)}₁ is selected with a wideband frequency-granularity while theother two matrices are selected per subband. By combining the two matrixfactors into one matrix factor, the PMI selection can be jointly encodedover the two factors which may reduce the PMI reporting overhead.

In a variation of this embodiment, separate inter-panel co-phasingcoefficients are used for each transmitted layer, so that the combinedper-panel may have the structure

${= {\left( {w_{PANEL} \otimes W_{2}} \right) = \begin{bmatrix}1 & 1 \\e^{j\phi} & {- e^{j\phi}} \\e^{j\alpha_{10}} & e^{j\beta_{10}} \\{e^{j\alpha_{10}}e^{j\phi}} & {{- e^{j\beta_{10}}}e^{j\phi}} \\e^{j\alpha_{01}} & e^{j\alpha_{01}} \\{e^{j\alpha_{01}}e^{j\phi}} & {{- e^{j\beta_{01}}}e^{j\phi}} \\e^{j\alpha_{11}} & e^{j\alpha_{11}} \\{e^{j\alpha_{11}}e^{j\phi}} & {{- e^{j\beta_{11}}}e^{j\phi}}\end{bmatrix}}},$

where the

e^(jα) ^(r) _(V) ^(r) _(H)

is the inter-panel co-phasing factor for panel (r_(V),r_(H)) for thefirst layer and

e^(jβ) ^(r) _(V) ^(r) _(H)

is the inter-panel co-phasing factor for panel (r_(V),r_(H)) for thesecond layer.

Analogue Beamforming Embodiments

The previously discussed embodiments are appropriate when each paneluses sufficiently many antenna ports, such as when a digital panelimplementation is used. However, if for instance an analogue panelimplementation is used instead, the number of antenna ports per panelmay be small. In that case, it may not be necessary to use a DFTprecoder within the panel (as the precoding across antenna elements ofthe same polarization in the panel is typically done in a UE transparentway when an analogue implementation is used, and thus, it may not haveto be included in the codebook). Thus, in some embodiments, themulti-panel precoder matrix consists of only two matrix factor, as

$W_{MP} = {{w_{PANEL} \otimes \left( W_{2} \right)} = {{\underset{\underset{\overset{\bigtriangleup}{=}\; W_{3}}{︸}}{\left( {w_{PANEL} \otimes I_{2}} \right)}W_{2}} = {W_{3}{W_{2}.}}}}$

It should be noted that the previously disclosed embodiments (comprisingthree matrix factors) can be combined with this embodiments as well,when applicable.

Details of Inter-Panel Co-Phasing Matrix Factor

The next section contains embodiments for how the inter-panel co-phasingmatrix factor w_(PANEL)/W₃ is selected.

In one embodiment, each co-phasing factor

e^(jα) ^(r) _(V) ^(R) _(H)

is selected independently of the other co-phasing factors for the otherpanels, so called scalar quantization. For instance,

e^(jα) ^(r) _(V) ^(R) _(H)

may be selected from one of the following sets: {1,−1}, or {1,j,−1,−j},corresponding to a B-PSK and Q-PSK constellation. In a more generalembodiment, the co-phasing factors may be selected from a 2^(K)_PSKconstellation, i.e. comprising the values

$\left\{ {1,e^{j\; 2\;{\pi \cdot \frac{1}{K}}}\ ,e^{j\; 2\;{\pi \cdot \frac{2}{K}}},\ldots\mspace{14mu},e^{j\; 2\;{\pi \cdot \frac{K - 1}{K}}}} \right\}.$

In another embodiment, vector quantization is used to select theinter-panel co-phasing. That is, w_(PANEL) is selected from a set ofpossible vectors w_(PANEL)∈{w_(A),w_(B), . . . }, i.e. w_(PANEL) isselected from an inter-panel codebook.

In one embodiment, applicable for only 4 antenna panels, the LTE 4TXRel-8 Householder rank-1 codebook (from 3GPP TS 36.211 Rel-8) is used asthe inter-panel codebook. That is w_(PANEL) may be selected from the 16possible values of W_(n) ^({1}), which is defined as: “The quantityW_(n) ^({s}) denotes the matrix defined by the columns given by the set{s}from the expression W_(n)=I−2_(u)u_(n) ^(H)/u_(n) ^(H)u_(n) where Iis the 4×4 identity matrix and the vector u_(n) is given by Table 1”

TABLE 1 Definition of 4TX codebook from 3GPP TS 36.211 Number of layersυ Codebook index u_(n) 1 2 3 4 0 u₀ = [1 −1 −1 −1]^(T) W₀ ^({1}) W₀^({14})/√{square root over (2)} W₀ ^({124})/√{square root over (3)} W₀^({1234})/2 1 u₁ = [1 −j 1 j]^(T) W₁ ^({1}) W₁ ^({12})/√{square rootover (2)} W₁ ^({123})/√{square root over (3)} W₁ ^({1234})/2 2 u₂ = [1 1−1 1]^(T) W₂ ^({1}) W₂ ^({12})/√{square root over (2)} W₂^({123})/√{square root over (3)} W₂ ^({3214})/2 3 u₃ = [1 j 1 −j]^(T) W₃^({1}) W₃ ^({12})/√{square root over (2)} W₃ ^({123})/√{square root over(3)} W₃ ^({3214})/2 4 u₄ = [1 (−1 −j)/√{square root over (2)} −j (1−j)/√{square root over (2)}]^(T) W₄ ^({1}) W₄ ^({14})/√{square root over(2)} W₄ ^({124})/√{square root over (3)} W₄ ^({1234})/2 5 u₅ = [1 (1−j)/√{square root over (2)} j (−1 −j/√{square root over (2)}]^(T) W₅^({1}) W₅ ^({14})/√{square root over (2)} W₅ ^({124})/√{square root over(3)} W₅ ^({1234})/2 6 u₆ = [1 (1 +j)/√{square root over (2)} −j (−1+j/√{square root over (2)}]^(T) W₆ ^({1}) W₆ ^({13})/√{square root over(2)} W₆ ^({134})/√{square root over (3)} W₆ ^({1324})/2 7 u₇ = [1 (−1+j)/√{square root over (2)} j (1 +j)/√{square root over (2)}]^(T) W₇^({1}) W₇ ^({13})/√{square root over (2)} W₇ ^({134})/√{square root over(3)} W₇ ^({1324})/2 8 u₈ = [1 −1 1 1]^(T) W₈ ^({1}) W₈ ^({12})/√{squareroot over (2)} W₈ ^({124})/√{square root over (3)} W₈ ^({1234})/2 9 u₉ =[1 −j −1 −j]^(T) W₉ ^({1}) W₉ ^({14})/√{square root over (2)} W₉^({134})/√{square root over (3)} W₉ ^({1234})/2 10 u₁₀ = [1 1 1 −1]^(T)W₁₀ ^({1}) W₁₀ ^({13})/√{square root over (2)} W₁₀ ^({123})/√{squareroot over (3)} W₁₀ ^({1324})/2 11 u₁₁ = [1 j −1 j]^(T) W₁₁ ^({1}) W₁₁^({13})/√{square root over (2)} W₁₁ ^({134})/√{square root over (3)} W₁₁^({1324})/2 12 u₁₂ = [1 −1 −1 1]^(T) W₁₂ ^({1}) W₁₂ ^({12})/√{squareroot over (2)} W₁₂ ^({123})/√{square root over (3)} W₁₂ ^({1234})/2 13u₁₃ = [1 −1 1 −1]^(T) W₁₃ ^({1}) W₁₃ ^({13})/√{square root over (2)} W₁₃^({123})/√{square root over (3)} W₁₃ ^({1324})/2 14 u₁₄ = [1 1 −1−1]^(T) W₁₄ ^({1}) W₁₄ ^({13})/√{square root over (2)} W₁₄^({123})/√{square root over (3)} W₁₄ ^({3214})/2 15 u₁₅ = [1 1 1 1]^(T)W₁₅ ^({1}) W₁₅ ^({12})/√{square root over (2)} W₁₅ ^({123})/√{squareroot over (3)} W₁₅ ^({1234})/2

In another, similar, embodiment, the set of vectors u_(n), n=0,1, . . ., 15 as defined in Table 1 is used as the inter-panel codebook. That is,the Householder transformation is not applied as we are only interestedin rank-1 vectors anyway.

In another embodiment, applicable to 3 antenna panels, the inter-panelcodebook (comprising length-3 vectors) is generated by taking theprecoders from the LTE 4TX Rel-8 Householder codebook and removing oneof the elements in each precoder (for instance removing the fourth row).

In another embodiment, a DFT precoder codebook is used for theinter-panel codebook.

Codebook Design for Non-Coherent Multi-Panel Transmission

For non-coherent transmission between panels, antenna portscorresponding to different panels are not coherently combined to formthe transmission of a single layer. Instead, different layers aretransmitted from each panel. This relaxes the requirement ofsynchronization between panels, and so, allows for a less compleximplementation.

A codebook for non-coherent transmission between panels can then beexpressed as a block-diagonal matrix (where the blocks may be ofdifferent sizes depending on how many layers are transmitted from eachpanel):

${\begin{bmatrix}W_{P00} \\W_{P10} \\W_{P01} \\W_{P11}\end{bmatrix} = {\begin{bmatrix}W_{1}^{00} & W_{2}^{00} & {\; 0} & 0 & 0 \\0 & W_{1}^{10} & W_{2}^{10} & {\; 0} & 0 \\0 & 0 & W_{1}^{01} & W_{2}^{01} & {\; 0} \\0 & 0 & 0 & W_{1}^{11} & W_{2}^{11}\end{bmatrix} = {{diag}\;\left( {{W_{1}^{00}W_{2}^{00}},{W_{1}^{10}W_{2}^{10}},{W_{1}^{01}W_{2}^{01}},{W_{1}^{11}W_{2}^{11}}} \right)}}},$

where W₁ ^(r) ^(V) ^(r) ^(H) W₂ ^(r) ^(V) ^(r) ^(H) may be a DFTprecoder. Note that in the above expression, all the antenna panels aretransmitting, and the rank is thus at least as large as the number ofpanels (each panel may then transmit several layers). In anotherembodiment, a panel selection component is added to the precoderstructure so that a subset of panels is selected in a first step:

${\begin{bmatrix}W_{P00} \\W_{P10} \\W_{P01} \\W_{P11}\end{bmatrix} = {\left( {\begin{bmatrix}i_{00} \\i_{10} \\i_{01} \\i_{11}\end{bmatrix} \otimes I_{NM}} \right) \cdot \begin{bmatrix}W_{1}^{00} & W_{2}^{00} & 0 & 0 & 0 \\0 & W_{1}^{10} & W_{2}^{10} & 0 & 0 \\0 & 0 & W_{1}^{01} & W_{2}^{01} & 0 \\0 & 0 & 0 & W_{1}^{11} & W_{2}^{11}\end{bmatrix}}},$

where i_(r) _(V) _(r) _(H) ∈{1,0} controls if a panel is on or off.

In another embodiment, the precoder matrix is factorized into two matrixfactors, where a first matrix factor comprises a set of orthogonal DFTbeams that are common for all the antenna panels and the second matrixfactor comprises per-panel precoding which may comprise DFT beamselection:

$\begin{bmatrix}W_{P00} \\W_{P10} \\W_{P01} \\W_{P11}\end{bmatrix} = {\left( {I_{N_{g}M_{g}} \otimes W_{1}} \right)\begin{bmatrix}W_{2}^{00} & {\; 0} & 0 & 0 \\0 & W_{2}^{10} & 0 & 0 \\0 & 0 & W_{2}^{01} & 0 \\0 & 0 & 0 & W_{2}^{11}\end{bmatrix}}$

where W₁ may for instance comprise two orthogonal DFT beams as

$W_{1} = \begin{bmatrix}{w_{2D}\left( {{l + O},m} \right)} & {w_{2\; D}\left( {l,m} \right)} & 0 & 0 \\0 & 0 & {w_{2D}\left( {{l + O},m} \right)} & {w_{2\; D}\left( {l,m} \right)}\end{bmatrix}$

where O is an oversampling factor and (l,m) is a 2D DFT beam index.

The W₂ matrix may then for instance comprise selection vectors, such as

$W_{2} = \begin{bmatrix}e_{k} & e_{k} \\{e^{j\phi_{1}}e_{k}} & {e^{j\phi_{2}}e_{k}}\end{bmatrix}$

where e_(k) is a selection vector which contains a 1 at row k and haszeros on all other rows.

Analogues Beamforming Embodiments

Similarly as for the coherent case, the non-coherent codebook may beused for analogue beamforming with a relatively small number of portsper panel as well. In this case, only two matrix factors may be neededand the resulting precoder matrix has the structure

$\begin{bmatrix}W_{P00} \\W_{P10} \\W_{P01} \\W_{P11}\end{bmatrix} = {\begin{bmatrix}W_{2}^{00} & {\; 0} & 0 & 0 \\0 & W_{2}^{10} & 0 & 0 \\0 & 0 & W_{2}^{01} & 0 \\0 & 0 & 0 & W_{2}^{11}\end{bmatrix} = {{{diag}\left( {W_{2}^{00},W_{2}^{10},W_{2}^{01},W_{2}^{11}} \right)}.}}$

I.e. the “per-panel DFT precoder matrix factor”, “W1”, may be removedfrom the expression.

Example Embodiments

-   -   1. A method performed by a UE for determining CSI feedback        corresponding to transmission from multiple antenna panels of a        transmitting network node, the method comprising:        -   a. receiving, from the network node, a signaling of the            antenna panel structure (i.e., a structural arrangement of            the multiple antenna panels at the transmitting network            node),        -   b. determining a multi-panel precoder codebook based on the            received signaling,        -   c. measuring a set of CSI-RS corresponding to multiple            antenna panels of the transmitting network node,        -   d. selecting at least one precoder matrix from the            multi-panel precoder codebook based on the measured set of            CSI-RS, and        -   e. transmitting at least one precoder matrix indication,            PMI, indicating the selection of the at least one precoder            matrix to the network node.    -   2. The method of embodiment 1, wherein the multi-panel precoder        codebook comprises precoder matrices, that        -   a. combines antenna ports corresponding to separate antenna            panels onto the same layers [coherent multi-panel            transmission], and or        -   b. combines antenna ports corresponding to separate antenna            panels onto separate layers [non-coherent multi-panel            transmission].    -   3. The method of embodiment 2, where the multi-panel precoder        codebook comprises precoder matrices comprising at least two        matrix factors and wherein applying one “inter-panel” matrix        factor comprises applying, for each antenna panel, a        panel-specific complex weight to all antenna ports corresponding        to that panel.    -   4. The method of embodiment 3, where another of the at least two        matrix factors comprise a DFT precoder matrix factor applied to        each antenna panel.    -   5. The method of embodiment 3, where the panel-specific complex        weight is a phase shift.    -   6. The method of embodiment 3, where the inter-panel matrix        factor is selected:        -   a. on a wideband basis,        -   b. on a per-subband basis.    -   7. The method of embodiment 1, where the received signaling of        the antenna panel structure comprises one or more of:        -   a. the number of antenna panels, N_(g)M_(g)        -   b. the number of antenna panels in each dimension N_(g),            M_(g)    -   8. The method of embodiment 5, where the panel-specific phase        shifts are selected independently per panel (scalar        quantization) from a {B,Q,8}-PSK constellation.    -   9. The method of embodiment 5, where the panel-specific phase        shifts are selected from a codebook that is:        -   a. the LTE 4TX rank-1 Householder codebook,        -   b. a subsampled and/or punctured LTE 4TX rank-1 Householder            codebook, or        -   c. a DFT codebook.    -   10. The method of embodiment 2, where the multi-panel precoder        codebook comprises precoder matrices that concatenates        independently selected per-panel precoders.    -   11. The method of embodiment 2, where the multi-panel precoder        codebook comprises precoder matrices comprising at least two        matrix factors and where a first matrix factor is common for        panels while a second matrix factor comprise per-panel        precoding.    -   12. The method of embodiment 1, where the measuring comprises        measuring on one or more CSI-RS resource per antenna panel and        selecting a preferred CSI-RS resource per panel and indicating        the selection as part of the CSI feedback report.

The following section comprises further considerations for extension ofType I CSI feedback to support multi-panel operation.

A motivation for using multiple panels instead of a fitting all theantenna elements into a single calibrated panel is to decrease theimplementation complexity. By its nature, multi-panel arrays are asuitable design when the gNB employs many antenna elements and TXRUs.Thus, the number of antenna elements per panel can be assumed to belarge and thus require a large number antenna ports per panel. However,as only 32 antenna ports for coherent combining are agreed to besupported thus far in NR, it is questionable if a multi-panelimplementation is warranted as this does not result in that many antennaports per panel, and so, a single-panel implementation should suffice ifcoherent transmission is the intended scheme. However, if up to 64antenna ports are supported, and/or non-coherent transmission isconsidered, a multi-panel implementation could be a viable alternativewhich would warrant explicit specification support.

Further, if antenna panels are uncalibrated with respect to carrierfrequency or sampling clock timing, coherent transmission between panelsmay be unfeasible since not only will a phase and amplitude offsetbetween panels be introduced after OFDM demodulation, ICI is introducedas well which limits the benefit of coherent transmission. While theresulting phase and amplitude scaling can be compensated for in aprecoder codebook, the ICI cannot be mitigated without explicitlyestimating the frequency and timing offset and compensating for them inthe OFDM (de)modulation. Thus, for coherent multi-panel transmission tobe beneficial, panels must be assumed to be well-enough calibrated(note, though, that a phase offset between panels will not introduce anyICI).

The default mode of operation for multi-panel arrays should thus beconsidered to be non-coherent joint transmission (JT) between thepanels.

For non-coherent transmission between panels, antenna portscorresponding to different panels are not coherently combined to formthe transmission of a single layer. Instead, the antenna ports of eachpanel are mapped to separate CSI-RS resources and different layers aretransmitted from each panel. This relaxes the requirement ofsynchronization between panels, and so, allows for a less compleximplementation.

From a UEs perspective, it should not fundamentally matter if themultiple layers in a non-coherent JT is transmitted from co-sited panelsbelonging to the same TRP or if they are transmitted from multiple TRPson different physical locations. Thus, non-coherent transmission betweenpanels should be handled in the same framework as non-coherenttransmission between TRPs, as is further elaborated in.

Multi-Panel Codebook Design for Coherent Transmission:

A DFT precoder codebook, such as the LTE Class A codebooks, comprisesprecoder vectors with linearly increasing phases over the antenna portsin each spatial dimension. Such a codebook design implicitly assumes anantenna setup of phase-calibrated and equally spaced antenna ports ineach dimension. In this case, the codebook perfectly matches the arrayresponse assuming a pure line-of-sight channel and gives a goodrepresentation of the dominant channel path for other propagationconditions. In the case of an uncalibrated multi-panel array, and/or, anon-uniform multi-panel array, the implicit assumptions of the DFTcodebook are thus broken. That is, applying a LTE Class A type DFTprecoder across antenna elements of the multiple panels may not resultin an efficient representation of the channel response. This is due toseveral factors:

The spacing between the last antenna element of a panel and the firstantenna element of the next panel is different from the antenna elementspacing within a panel for a non-uniform panel array. Thus, the phaseshift between said antenna elements would have to be

$e^{\frac{j2\pi{k{({1 + \Delta_{pa{nel}}})}}}{N}}$

rather than

$e^{\frac{j2\pi k}{N}}$

(as it is for the DFT precoder) in order to create a linear phase front,where k is the DFT precoder index, N the number of antennas in adimension and Δ_(panel) the additional distance between panels comparedto the distance between panels in a uniform multi-panel array. Thisphase difference could of course be compensated directly for in thecodebook if the panel distance was known (thus avoiding the introductionof an additional codebook component), however, the distance betweenpanels is up to implementation.

There may exist an additional phase offset between panels due to, forinstance, different LO phase state or frequency offset. In the worstcase, the phase offset may be completely random and thus uniformlydistributed in [0, 2π].

If the antenna panels have a timing misalignment, this may introduce afrequency-selective phase offset.

We first note that first two phase offsets do not depend on frequency,and thus, the compensation may be done on a wideband basis. We then notethat for Type I single-beam CSI feedback, the first phase offset neednot be explicitly compensated for as the second phase offset anyway canbe uniformly distributed in [0, 2π]. However, for Type II CSI feedbackwith beam combination, the first phase offset should be modelledexplicitly since it is different for each beam and thus, a separatephase compensation factor for each beam and panel may be needed.

To compensate for a frequency-selective phase offset due to a possibletiming misalignment, a per-subband co-phasing of antenna panels may beneeded. This would however incur a large amount of overhead incomparison to the performance that can be expected. Essentially, onewould pay the overhead for Type II feedback while only achieving Type Iperformance. In our view, this overhead is not warranted and any timingmisalignment should be solved by gNB implementation rather than beincorporated in UE feedback.

As an observation, a wideband panel co-phasing can be used to compensatefor non-uniform panel spacing and different LO phase state; afrequency-selective panel co-phasing is needed to compensate for timingmisalignment between panels.

For the precoding within a panel, the same codebook as for thesingle-panel case should be used. That is, the regular “LTE ClassA”-like codebook with W1W2 structure, where W1 comprises beam selectionand W2 comprises polarization cophasing should be used. As the antennapanels should be co-located for coherent transmission, the panels shouldsee the same propagation environment, and thus, the same selection of W1and W2 should be optimal for all panels.

If the panel uses analogue beamforming rather than a digitalimplementation, a “LTE Class B”-like codebook with W2 only could be usedas the per-panel codebook.

As a proposal:

a uniformly distributed wideband phase offset between panels should becompensated for in the coherent multi-panel codebookThe precoding within a panel should use the single-panel codebookFrequency-selective panel cophasing is not supportedBased on these proposals, we can design the coherent multi-panelcodebook.

For ease of explanation, assume that M_(g)=N_(g)=2 so that 4 antennapanels are used, as is illustrated in Error! Reference source not found.Assume also in the following that the antenna elements of the panelarray are indexed with

i=M _(g)PNM·r _(V)+PNM·r _(v) +NM·p+M·n+m,

Where r_(V)=0, . . . , M_(g)−1 is the vertical panel index, r_(H)=0, . .. , N_(g)−1 is the horizontal panel index, m=0, . . . , M−1 is thevertical antenna port index within a panel, n=0, . . . , N−1 is thehorizontal antenna port index within a panel and p=0,1 is thepolarization index. That is, the multi-panel precoding matrix W_(MP) isconstructed by stacking the precoding matrices for the constituentpanels W_(P00), W_(P10), W_(P01), W_(P11) so that

$W_{MP} = {\begin{bmatrix}W_{P00} \\W_{P10} \\W_{P01} \\W_{P11}\end{bmatrix}.}$

The inter-panel phase offset compensation can be described by thelength-M_(g)N_(g) vector

${w_{PANEL} = \begin{bmatrix}1 \\e^{j\alpha_{10}} \\e^{j\alpha_{01}} \\e^{j\alpha_{11}}\end{bmatrix}},$

where

e^(jα) ^(r) _(V) ^(r) _(H)

is the phase compensation for panel (r_(V),r_(H)). Assuming the sameselection of W₁ and W₂ can be made for all panels, the multi-panelprecoder matrix may be expressed as

$W_{MP} = {{w_{PANEL} \otimes \left( {W_{1}W_{2}} \right)} = {{\underset{\underset{\overset{\bigtriangleup}{=}\; W_{3}}{︸}}{\left( {w_{PANEL} \otimes I_{2{MN}}} \right)}W_{1}W_{2}} = {W_{3}W_{1}{W_{2}.}}}}$

Proposal: For coherent Type I multi-panel codebook, consider using aternary W_3 W_1 W_2 codebook structure where,

-   -   W₃=w_(PANEL)⊗I_(2MN) and

$w_{PANEL} = \begin{bmatrix}1 \\e^{j\alpha_{10}} \\\vdots \\e^{j\alpha_{{({M_{g}{­1}})}{({N_{g}{­1}})}}}\end{bmatrix}$

selected on a wideband basis

-   -   W₁W_(z) is according to the single-panel codebook        Note: W₁=I in case of M=N=1 for e.g. analogue panels

Some options to design the codebook for w_(PANEL)/W₃:

-   -   Scalar Quantization: Each element of w_(PANEL) is encoded        independently and chosen from a PSK constellation. This will        result in the best performance but will result in larger W₃        overhead.    -   Vector Quantization: The elements in w_(PANEL) are jointly        encoded and selected from a codebook.        -   Unstructured codebook: As the phase offsets between panels            should be uncorrelated, an unstructured codebook, such as            the LTE 4TX Householder rank-1 codebook, should work well.        -   2D-DFT codebook: For reference, should perform worse than            the unstructured codebook.

Evaluation Results of Coherent Multi-Panel Codebook:

In the following simulation results comparing the different W₃-codebookdesigns are discussed. As a reference and baseline for comparison, wealso evaluate the LTE Rel-13/14 DFT codebook with M_(g)M×N_(g)N portlayout applied across the multiple panels. The evaluated W₃-codebookdesigns, as well as the associated overhead, are presented in In thefollowing table. For all multi-panel codebooks, the LTE Rel-13/14 W1W2codebook with M×N port layout is applied per panel.

The multi-panel antenna with four 4×4 panels as illustrated in FIG. 10has been used in the simulations. A 2×2 subarray virtualization has beenapplied per panel, so that each panel comprises 8 ports, meaning that 32ports is used in total. Performance has been evaluated in the 3GPP 3DUMi (Urban Micro) scenario using the FTP1 traffic model with 100 kBpacket size. Remaining simulation parameters are listed in the Appendix.

In the following table, overhead for W1 and W3 for the differentcodebooks is listed:

Codebook W1 + W3 overhead LTE Rel-13 Class O = 4 8 bits A CB O = 8 10bits  O = 16 12 bits Scalar BPSK 9 bits Quantization W3 QPSK 12 bits 4TXHouseholder W3 10 bits 2D-DFT W3 O = 1 8 bits O = 2 10 bits O = 4 12bits

Evaluation results are presented in the table below, depicting aperformance of different codebooks in 3GPP 3D UMi scenario at 50% RU. Asseen, applying the “Rel-13”-like codebook across panels is not veryefficient and results in relatively poor performance compared to themulti-panel codebooks. Of the multi-panel codebooks, the scalarquantization W3 codebook of course performs the best, even whenconsidering the overhead. The Householder W3 codebook gives slightlylarger gains than the 2D-DFT codebook when compared at the sameoverhead.

NORMALISED NORMALIZED CELL EDGE USER CELL USER THROUGHPUT THROUGHPUTEDGE THROUGHPUT [BPS/HZ/USER] [BPS/HZ/USER] GAIN [%] GAIN [%] REL-13 CBO = 4 0.50592 2.3138 0 0 REL-13 CB O = 8 0.51976 2.34 3 1 REL-13 CB O =12 0.53126 2.3562 5 2 PANEL CB SCALAR 0.59407 2.4833 17 7 QUANTIZATIONBPSK PANEL CB SCALAR 0.72087 2.7172 42 17 QUANTIZATION QPSK PANEL CBREL-8 0.6104 2.53 21 9 HOUSEHOLDER PANEL CB DFT O = 1 0.49045 2.2937 −3−1 PANEL CB DFT O = 2 0.59376 2.4909 17 8 PANEL CB DFT O = 4 0.63752.5683 26 11

Observations:

-   -   Applying a Rel-13 like codebook across the multiple antenna        panels results in relatively poor performance, increasing        oversampling factor does not increase codebook performance    -   Scalar quantization of panel cophasing yields best performance    -   Householder codebook performs better than DFT at the same        overhead

As the differences in feedback overhead between the investigated schemesis only a few wideband bits, it makes sense to go for the bestperforming scheme. The following may be proposed:

-   -   For w_(PANEL) codebook, consider using scalar quantization of        panel cophasing coefficients with QPSK constellation.

SOME CONCLUSIONS

With respect to an applicability of multi-panel operation, as well asproposed and evaluated codebook designs for coherent multi-paneltransmission, following observations may be considered:

-   -   It is not clear if explicit multi-panel support is required if        NR only supports up to 32 antenna ports    -   If frequency and/or timing offset exists between panels, the        induced ICI can prohibit coherent transmission between panels    -   A wideband panel co-phasing can be used to compensate for        non-uniform panel spacing and different LO phase state.    -   A frequency-selective panel co-phasing is needed to compensate        for timing misalignment between panels.    -   Applying a Rel-13 like codebook across the multiple antenna        panels results in relatively poor performance, increasing        oversampling factor does not increase codebook performance    -   Scalar quantization of panel cophasing yields best performance    -   Householder codebook performs better than DFT at the same        overhead

Based on these observations, the following proposals may be derived:

-   -   A uniformly distributed wideband phase offset between panels        should be compensated for in the coherent multi-panel codebook    -   The precoding within a panel should use the single-panel        codebook a Frequency-selective panel cophasing is not supported    -   For coherent Type I multi-panel codebook, consider using a        ternary W₃W₁W₂ codebook structure where,    -   W₃=w_(panel)⊗I_(2MN) and

$w_{PANEL} = \begin{bmatrix}1 \\e^{j\alpha_{10}} \\\vdots \\e^{j\alpha_{{({M_{g}{­1}})}{({N_{g}{­1}})}}}\end{bmatrix}$

selected on a wideband basis

-   -   W₁W₂ is according to the single-panel codebook    -   Note: W₁=I in case of M=N=1 for e.g. analogue panels    -   For w_(PANEL) codebook, consider using scalar quantization of        panel cophasing coefficients with QPSK constellation

Simulation Parameters:

Carrier frequency 2 GHz Bandwidth 10 MHz Scenarios 3D UMi 200 m ISDAntenna Configurations Non-Uniform Panel Array M_(g) = N_(g) = 2, M = N= 4, 2x2 virtualizaiton 130° tilt 0.8λ vertical antenna spacing, 0.5λhorizontal antenna spacing Panel spacing 2x that of uniform panel array(4λ horizontally, 6.4λ vertically) Random phase error between panelsCell layout 57 homogeneous cells Wrapping Radio distance based UEreceiver MMSE-IRC CSI periodicity 5 ms CSI delay 5 ms CSI mode PUSCHMode 3-2 Outer loop Link Adaptation Yes, 10% BLER target UE noise figure9 dB eNB Tx power 41 dBm (UMi) Traffic model FTP Model 1, 100 kB packetsize UE speed 3 km/h Scheduling Proportional fair in time and frequencyDMRS overhead 2 DMRS ports CSI-RS Overhead accounted for. Channelestimation error modeled. HARQ Max 5 retransmissions Handover margin 3dB Transmission Mode SU-MIMO

What is claimed is:
 1. A method performed by a wireless communicationdevice configured for use in a wireless communication system, the methodcomprising the wireless communication device: determining, based on oneor more structural properties of a multi-panel antenna array describinghow the antenna array is structured into multiple panels, a precoder torecommend to a transmit radio node for applying to a transmission fromthe multi-panel antenna array, wherein the multiple panels include atleast some panels that have identical antenna port layouts andintra-panel antenna port indices; signaling the determined precoder tothe transmit radio node; and wherein the determined precoder applies thesame phase offset between antenna ports that correspond to spatiallyadjacent panels and that have the same intra-panel antenna port indices.2. The method of claim 1: wherein the one or more structural propertiesinclude a total number of the multiple panels into which the antennaarray is structured; and wherein the determined precoder is configuredto apply, for each of the multiple panels, a panel-specific complexweight to all antenna ports corresponding to that antenna panel.
 3. Themethod of claim 1, wherein the panel-specific complex weight is a phaseshift with unitary amplitude.
 4. The method of claim 1, wherein the oneor more structural properties include, for each of one or more spatialdimensions in which the multiple panels are arranged, a number of panelsinto which the antenna array is structured in that spatial dimension. 5.The method of claim 1, wherein the one or more structural propertiesinclude a spatial arrangement of the multiple panels into which theantenna array is structured.
 6. The method of claim 1, furthercomprising receiving signaling indicating the one or more structuralproperties.
 7. The method of claim 6, wherein the signaling is radioresource control signaling.
 8. The method of claim 6, wherein thesignaling is received during a procedure for configuring a channel stateinformation process or channel state information reference signalresource.
 9. The method of claim 6, wherein the signaling is physicallayer signaling; wherein the signaling is included in a downlink controlinformation message on a physical downlink control channel.
 10. Themethod of claim 6, wherein the signaling is included in a message thatconveys an uplink grant for the wireless communication device totransmit a channel state information feedback report to the transmitradio node.
 11. The method of claim 1, wherein the determining comprisesdetermining a multi-panel precoding codebook based on the one or morestructural properties.
 12. The method of claim 11, wherein thedetermining the multi-panel precoding codebook comprises determining themulti-panel precoding codebook as being one of multiple differentpossible precoding codebooks respectively corresponding to differentpossible ways in which the antenna array is structurable into multiplepanels.
 13. The method of claim 11, wherein the determining themulti-panel precoding codebook comprises modifying a predefinedprecoding codebook based on the one or more structural properties. 14.The method of claim 11, wherein the determining further comprisesselecting the precoder from the determined multi-panel precodingcodebook, based on measurement of one or more reference signalstransmitted from the multi-panel antenna array.
 15. The method of claim1, wherein the precoder is determined from an inter-panel precodingcodebook, wherein the inter-panel precoding codebook is a DiscreteFourier Transform (DFT) codebook, and further comprising signaling: anindex indicating a recommended intra-panel precoder selected from anintra-panel precoding codebook; and/or an index indicating a recommendedpolarization co-phasing precoder selected from a polarization co phasingprecoding codebook.
 16. The method of claim 1, wherein the determinedprecoder co-phases panels of the multi-panel antenna array.
 17. Themethod of claim 1, wherein the determined precoder applies a phaseoffset between antenna ports corresponding to different panels thatdiffers from a phase offset applied to antenna ports corresponding tothe same panel.
 18. The method of claim 1, wherein the determinedprecoder is configured to combine antenna ports corresponding todifferent antenna panels onto the same transmission layer.
 19. Themethod of claim 1, wherein determining the precoder comprisesdetermining, independently for each of the multiple panels, thepanel-specific complex weight that is to be applied to all antenna portscorresponding to that panel.
 20. The method of claim 1, wherein theprecoder is determined based on measurement of one or more channel stateinformation reference signals transmitted from the multi-panel antennaarray on a set of one or more channel state information reference signalresources.
 21. A method implemented by a transmit radio node configuredfor transmitting via a multi-panel antenna array in a wirelesscommunication system, the method comprising the transmit radio node:transmitting, to a wireless communication device, signaling indicatingone or more structural properties of a multi-panel antenna arraydescribing how the antenna array is structured into multiple panels,wherein the one or more structural properties include a total number ofthe multiple panels into which the antenna array is structured, andwherein the multiple panels include at least some panels that haveidentical antenna port layouts and intra-panel antenna port indices; andreceiving, responsive to transmitting the signaling and from thewireless communication device, signaling indicating a precoder that thewireless communication device recommends to the transmit radio node forapplying to a transmission from the multi-panel antenna array; andwherein the determined precoder applies the same phase offset betweenantenna ports that correspond to spatially adjacent panels and that havethe same intra-panel antenna port indices.
 22. The method of claim 21,wherein: the determined precoder is configured to apply, for each of themultiple panels, a panel-specific complex weight to all antenna portscorresponding to that antenna panel; and the panel-specific complexweight is a phase shift with unitary amplitude.
 23. The method of claim21, wherein the one or more structural properties include, for each ofone or more spatial dimensions in which the multiple panels arearranged, a number of panels into which the antenna array is structuredin that spatial dimension.
 24. The method of claim 21, wherein the oneor more structural properties include a spatial arrangement of themultiple panels into which the antenna array is structured.
 25. Themethod of claim 21, further comprising: determining a precoder based onthe precoder recommended to the transmit radio node; and precoding atransmission from the multi-panel antenna array using the determinedprecoder.
 26. The method of claim 25 wherein the precoder is determinedfrom an inter-panel precoding codebook, and further comprising:receiving, from the wireless communication device, signaling indicating:an index indicating a recommended intra-panel precoder selected from anintra-panel precoding codebook; and/or an index indicating a recommendedpolarization co-phasing precoder selected from a polarization co phasingprecoding codebook.
 27. The method of claim 25, wherein the determinedprecoder co-phases panels of the multi-panel antenna array.
 28. Themethod of claim 25, wherein the determined precoder applies a phaseoffset between antenna ports corresponding to different panels thatdiffers from a phase offset applied to antenna ports corresponding tothe same panel.
 29. A wireless communication device configured for usein a wireless communication system, the wireless communication devicecomprising: processing circuitry; memory containing instructionsexecutable by the processing circuitry whereby the wirelesscommunication device is configured to: determine, based on one or morestructural properties of a multi-panel antenna array describing how theantenna array is structured into multiple panels, a precoder torecommend to a transmit radio node for applying to a transmission fromthe multi-panel antenna array, wherein the multiple panels include atleast some panels that have identical antenna port layouts andintra-panel antenna port indices; signal the determined precoder to thetransmit radio node; and wherein the determined precoder applies thesame phase offset between antenna ports that correspond to spatiallyadjacent panels and that have the same intra-panel antenna port indices.30. A transmit radio node configured for use in transmitting via amulti-panel antenna array in a wireless communication system, thetransmit radio node comprising: processing circuitry; memory containinginstructions executable by the processing circuitry whereby the transmitradio node is configured to: transmit, to a wireless communicationdevice, signaling indicating one or more structural properties of amulti-panel antenna array describing how the antenna array is structuredinto multiple panels, wherein the one or more structural propertiesinclude a total number of the multiple panels into which the antennaarray is structured; receive, responsive to the transmitting thesignaling and from the wireless communication device, signalingindicating a precoder that the wireless communication device recommendsto the transmit radio node for applying to a transmission from themulti-panel antenna array; and wherein the determined precoder appliesthe same phase offset between antenna ports that correspond to spatiallyadjacent panels and that have the same intra-panel antenna port indices.